1. Field of the Invention
The present invention relates to a display apparatus having a light emitting element driven by an active matrix method and a drive control method for the display apparatus.
2. Description of the Related Art
Display devices using light emitting elements, such as organic EL elements, for use in various applications, including televisions, cell phone displays, and the like, have been proposed.
Generally, organic EL elements are current driven light emitting elements and, unlike a liquid crystal display, require, as minimum, selection transistors for selecting pixel circuits, holding capacitors for holding charges according to an image to be displayed, and drive transistors for driving the organic EL elements as the drive circuit as described, for example, U.S. Pat. No. 5,684,365 (Patent Document 1).
Heretofore, thin film transistors of low-temperature polysilicon or amorphous silicon have been used in pixel circuits of active matrix organic EL display devices.
The low-temperature polysilicon thin film transistor may provide high mobility and stability of threshold voltage, but has a problem that the mobility is not uniform. The amorphous silicon thin film transistor may provide uniform mobility, but has a problem that the mobility is low and threshold voltage varies with time.
The non-uniform mobility and instable threshold voltage appear as irregularities in the displayed image. Consequently, for example, Japanese Unexamined Patent Publication No. 2003-255856 (Patent Document 2) proposes a display device in which a compensation circuit of diode connection method is provided in the pixel circuit.
The provision of the compensation circuit described in Patent Document 2, however, causes the pixel circuit to become complicated, resulting in increased cost due to low yield rate and low aperture ratio.
As such, for example, Japanese Unexamined Patent Publication Nos. 2002-278513 (Patent Document 3) and U.S. Patent Application Publication No. 20070210996 (Patent Document 4) propose a method in which a current meter is provided outside of the active matrix substrate, on which pixel circuits are disposed, with respect to each pixel circuit row to measure a current of each drive transistor by the current meter, then characteristic values of each drive transistor, including the threshold voltage, mobility, and the like, are calculated based on the measured drive current value and stored, and correction data are programmed into each pixel circuit as the gate voltage of each drive transistor based on the characteristic values, thereby achieving both the simplicity of pixel circuits and characteristic correction of drive transistors.
The method described in Patent Document 3 and Patent Document 4, however, can not measure the drive current accurately because the extinction current of an organic EL element of a non-selected pixel circuit gets into the measured drive current. Further, the method measures a very small drive current for one pixel circuit and has a problem in the measurement accuracy of the current from a practical viewpoint. Still further, the method can not perform the acquisition of correction data and display operation at the same time since it requires time for the measurement of drive currents, so that real time update of the correction data is impossible.
In the mean time, as for methods for correcting a characteristic variation of a drive transistor within the pixel circuit, a correction method with a simpler pixel circuit configuration is proposed as described, for example, in U.S. Patent Application Publication No. 20070268210 (Patent Document 5).
The correction method described in Patent Document 5 is a method in which the threshold voltage of a drive transistor is detected by charging a parasitic capacitance of the organic EL element, then a voltage variation is converted to the deviation of mobility μ, and the gate-source voltage to be supplied to the drive transistor is automatically corrected.
The method described in Patent Document 5, however, needs to perform control of rising and falling slopes of data signals in order to cover deviations in the parasitic capacitances of organic EL elements and fact that μ correction current differs each time according to the image data, and to perform correction for the influence of the resistance and capacitance of data lines. That is, the simplicity of pixel circuits is achieved at the expense of complicated drive control, requiring the drive control circuit to have an extraordinary accuracy so that the overall cost of the display apparatus is increased.
Further, U.S. Pat. No. 7,358,941 (Patent Document 6) proposes a method in which a wiring capacitance is used instead of charging the parasitic capacitance of an organic EL element as in Patent Document 5, and the voltage of the wiring capacitance is read by the drive circuit, whereby the properties of the drive transistor are corrected.
In the method described in Patent Document 6, although the measurement of a very small drive current, which is the problem of the method described in Patent Document 3 and Patent Document 4, can be realized by a simple voltage measurement, but it takes a long time to acquire correction data because it uses the wiring capacitance of the common potential line as the load capacitance.
In view of the circumstances described above, it is an object of the present invention to provide a display apparatus and a drive control method of the display apparatus capable of realizing accurate correction of characteristic deviations of drive transistors, simultaneous display operation and acquisition of characteristic values, and simplified pixel circuits and drive control of the circuits.